Tennis authorities have encountered "challenges" over the arrival dates of players in Australia ahead of the new 2021 season, the men's ATP Tour said, throwing the ATP Cup and other Australian Open leadup events into doubt.

Tennis Australia (TA) had planned for players to arrive in the southern state of Victoria by mid-December to give them time to undertake a mandatory two-week quarantine before competing but the ATP said that was now under review.

"In discussions with Tennis Australia over the past 24 hours, we have been informed there are some new challenges around the previously planned arrival dates for players and team members," the ATP said in note to players.

"We continue to work with Tennis Australia on confirming plans for January, and we will provide an update as soon as more information is available in the coming days.

"We understand there is uncertainty about the start of the 2021 season, and we are working as hard as possible to deliver the best possible calendar of events to players."

TA boss Craig Tiley told an Australian newspaper earlier this week that the governing body had decided to shift the team-based ATP Cup and all of the Australian Open warmup events to Victoria to mitigate the risk of logistical problems arising from Covid-19 protocols in different states.

But Tiley's comments were given short shrift by Victoria premier Daniel Andrews, who said his government had not even signed off on the 18-31 January Australian Open.

Australia has largely brought Covid-19 to heel but the nation is on high alert after a fresh outbreak in South Australia state in recent days, which has prompted neighbouring states to tighten their borders.

World number two Rafa Nadal said arriving later in Australia would not be a "big difference" as long as players could train while in quarantine.

"The only negative thing is we have to be there like 16 days in advance (of competing)," he told reporters at the ATP Finals on Tuesday.

World number one Novak Djokovic said on Monday he was still in the dark as to whether he would be able to lead Serbia's title defence at the team-based ATP Cup.